JP7470753B2 - Thermal recording medium - Google Patents

Description

This invention relates to a thermal recording medium that utilizes the color-developing reaction between a colorless or light-colored electron-donating leuco dye (hereinafter also referred to as "leuco dye") and an electron-accepting developer (hereinafter also referred to as "developer"), and that has excellent high-speed printability, oil resistance, plasticizer resistance, print running properties, etc.

Generally, a thermal recording medium is prepared by coating a support such as paper, synthetic paper, film, plastic, etc. with a coating liquid containing a colorless or light-colored leuco dye and a color developer, and a recorded image is obtained by instantaneous chemical reaction caused by heating with a thermal head, hot stamp, heat pen, laser light, etc. Thermal recording media are widely used as recording media for facsimiles, computer terminal printers, automatic ticket vending machines, measurement recorders, receipts at supermarkets and convenience stores, etc.
In recent years, the use of thermal recording media has expanded to include a wide variety of applications, such as for various tickets, receipts, labels, bank ATMs, gas and electricity meter reading, and vouchers for car and horse racing betting. As a result, a variety of performance properties are required, such as water resistance, resistance to plasticizers in the image areas, heat resistance and oil resistance in the blank areas, and storage stability of the image and blank areas under harsh conditions.
In response to such demands, there have been disclosed a thermosensitive recording medium (Patent Document 1) in which two specific types of color developers are combined to improve water resistance, plasticizer resistance in the image area, and heat resistance in the blank area, and a urea compound (Patent Documents 2 and 3) as a color developer for improving required performance such as color density, whiteness, and storage stability of the printed area of the thermosensitive recording medium.
Furthermore, as a method for improving the sensitivity and printing quality of a thermosensitive recording medium, it has been proposed to provide an undercoat layer containing hollow particles between the support and the thermosensitive recording layer (Patent Documents 4 and 5, etc.).

Patent Publication 2015-80852 International Publication WO2019/044462 Patent Publication No. 2020-066148 Patent Publication No. 2020-152027 Patent 6782511

The present invention aims to provide a thermal recording medium that, among the various properties required of a thermal recording medium, excels in high-speed printing, as well as in oil resistance, plasticizer resistance, print running properties, etc.

（式中、Ｒ^２は、水素原子又はアルキル基を表し、Ｒ^３は、それぞれ同じであっても異なってもよく、下式（化１１）で表される基であり、一般式（化５）のベンゼン環中のＲ^３－ＳＯ_２－Ｏ－の位置は、同じであっても異なってもよく、３位、４位又は５位である。）

（式中、Ｒ^４～Ｒ^８は、それぞれ同じであっても異なってもよく、水素原子、ハロゲン原子、ニトロ基、アミノ基、アルキル基、アルコキシ基、アリールオキシ基、アルキルカルボニルオキシ基、アリールカルボニルオキシ基、アルキルカルボニルアミノ基、アリールカルボニルアミノ基、アルキルスルホニルアミノ基、アリールスルホニルアミノ基、モノアルキルアミノ基、ジアルキルアミノ基、又はアリールアミノ基を表す。）
（２）前記第２のウレア化合物は下記一般式（化３）で表される、

（式中、Ｒ^２、Ｒ^４～Ｒ^８は、上記と同様に定義され、ｍは０～２の整数を表す。）、但し、前記第２のウレア化合物は下記一般式（化７）で表される化合物を除く。

As a result of extensive research, the inventors have discovered that the above problems can be solved by incorporating a specific urea compound as a developer in a thermosensitive recording layer provided on a support, providing an undercoat layer between the support and the thermosensitive recording layer, and incorporating a specific amount of hollow plastic particles in this undercoat layer, thereby completing the present invention.
That is, the present invention relates to a thermosensitive recording medium having an undercoat layer provided on a support, and a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer provided on the undercoat layer, wherein the thermosensitive recording layer contains, as the electron-accepting developer, a first urea compound represented by the following (1) or a second urea compound represented by the following (2), the undercoat layer contains a binder and a pigment, the pigment being contained in an amount of 50 to 95% by solid content, and the pigment is plastic hollow particles, the plastic hollow particles being contained in the pigment in an amount of 50% by solid content or more, the total content (solid content) of the first and second urea compounds in the thermosensitive recording layer is 1.0 to 70.0% by weight, the content (solid content) of the first urea compound in the thermosensitive recording layer is 1.0 to 50.0% by weight, and the content (solid content) of the second urea compound is 5.0 to 50.0% by weight.
(1) The first urea compound is represented by the following general formula (Chemical Formula 5):

(In the formula, ^R2 represents a hydrogen atom or an alkyl group, ^R3 may be the same or different and is a group represented by the following formula (Chemical formula 11), and the position of ^R3 - _SO2 -O- in the benzene ring of general formula (Chemical formula 5) may be the same or different and is the 3rd, 4th or 5th position.)

(In the formula, R ⁴ to R ⁸ may be the same or different and each represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group.)
(2) The second urea compound is represented by the following general formula (Chemical Formula 3):

(In the formula, R ² , R ⁴ to R ⁸ are defined as above, and m represents an integer of 0 to 2) , with the proviso that the second urea compound does not include a compound represented by the following general formula (Chemical Formula 7).

The present invention can provide a thermal recording medium that has good color development performance and good high-speed printability, and further has good oil resistance, plasticizer resistance, print running properties, etc.

The thermal recording medium of the present invention has a thermal recording layer on a support and an undercoat layer between the support layer and the thermal recording layer, the thermal recording layer containing a specific urea compound as an electron-accepting developer, and the undercoat layer containing a specific amount of plastic hollow particles.
Examples of various materials that can be used in the thermal recording layer of the thermal recording material of the present invention are given below, but binders, crosslinking agents, pigments, etc. can also be used in the undercoat layer and each coating layer provided as necessary, to the extent that they do not impede the desired effects for the above-mentioned problems.

（式中、Ｒ^２及びＲ^３は、上記と同様に定義される。）
（２）下記一般式（化３）で表される第２のウレア化合物、

（式中、Ｒ^２及びｍは上記と同様に定義され、Ｒ^４～Ｒ^８は後述する。）
（３）下式（化４）で表される第３のウレア化合物

（式中、Ｒ^２は上記と同様に定義され、Ｒ^４～Ｒ^８は後述する。） The urea compound of the present invention is selected from the following (1) and (2).
(1) A first urea compound represented by the following general formula (Chemical Formula 5):

(In the formula, ^R2 and ^R3 are defined as above.)
(2) A second urea compound represented by the following general formula (Chemical Formula 3):

(In the formula, R ² and m are defined as above, and R ⁴ to R ⁸ are described below.)
(3) A third urea compound represented by the following formula (Chemical Formula 4):

(In the formula, R ² is defined as above, and R ⁴ to R ⁸ are described below.)

Furthermore, the urea compound used in the present invention is selected from the urea compounds represented by (1) to (2) above.

The first urea compound used in the present invention is represented by the following formula (Formula 5).

In the general formula (5), n represents 0 or 1, and preferably 1.
In the general formula (Chemical Formula 5), R ³ represents an alkyl group, an aralkyl group, or an aryl group, which may be substituted or unsubstituted. The alkyl group is, for example, a linear, branched, or alicyclic alkyl group, and preferably has 1 to 12 carbon atoms. The aralkyl group preferably has 7 to 12 carbon atoms, and the aryl group preferably has 6 to 12 carbon atoms. When these are substituted, the substituent is preferably an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, or a halogen atom. Furthermore, multiple R ^3s may be the same or different.
The positions of R ³ —SO ₂ —O— in the benzene ring of the general formula (5) may be the same or different, and are preferably the 3-position, 4-position or 5-position.

Examples of the alkyl group include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, cyclopentyl, hexyl, cyclohexyl, 2-ethylhexyl, and lauryl.

Examples of the aralkyl group include benzyl, 1-phenylethyl, 2-phenylethyl, 3-phenylpropyl, p-methylbenzyl, m-methylbenzyl, m-ethylbenzyl, p-ethylbenzyl, p-i-propylbenzyl, p-t-butylbenzyl, p-methoxybenzyl, m-methoxybenzyl, o-methoxybenzyl, m,p-di-methoxybenzyl, p-ethoxy-m-methoxybenzyl, p-phenylmethylbenzyl, p-cumylbenzyl, p-phenylbenzyl, o-phenylbenzyl, m-phenylbenzyl, p-tolylbenzyl, m-tolylbenzyl, o-tolylbenzyl, and p-chlorobenzyl groups, and the like.

Examples of the aryl group include phenyl, p-tolyl, m-tolyl, o-tolyl, 2,5-dimethylphenyl, 2,4-dimethylphenyl, 3,5-dimethylphenyl, 2,3-dimethylphenyl, 3,4-dimethylphenyl, mesitylene, p-ethylphenyl, p-i-propylphenyl, p-t-butylphenyl, p-methoxyphenyl, 3,4-dimethoxyphenyl, p-ethoxyphenyl, p-chlorophenyl, 1-naphthyl, 2-naphthyl, and t-butylated naphthyl, and other unsubstituted or aryl groups substituted with alkyl groups, alkoxy groups, aralkyl groups, aryl groups, or halogen atoms.

^R2 represents a hydrogen atom or an alkyl group, preferably a hydrogen atom. The alkyl group is preferably an alkyl group having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, or a t-butyl group.
The positions of ^R2 in the benzene ring of the general formula (Chemical Formula 2) may be the same or different, and are preferably the 3-position, 4-position or 5-position.

一般式（化６）中、Ｒ^９はアルキル基又はアルコキシ基、好ましくはアルキル基であり、ｏは０～３、好ましくは０～２、より好ましくは０～１の整数を表す。このアルキル基の炭素数は、例えば、１～１２、好ましくは１～８、より好ましくは１～４である。
一般式（化６）のベンゼン環中のＲ^９の位置は、同じであっても異なってもよく、好ましくは３位、４位又は５位、好ましくは４位である。 As the first urea compound of the present invention, a urea compound represented by the following general formula (Formula 6) is more preferable.

In the general formula (Chemical Formula 6), ^R9 is an alkyl group or an alkoxy group, preferably an alkyl group, and o represents an integer of 0 to 3, preferably 0 to 2, more preferably 0 to 1. The number of carbon atoms in this alkyl group is, for example, 1 to 12, preferably 1 to 8, more preferably 1 to 4.
The positions of R ⁹ in the benzene ring of the general formula (6) may be the same or different, and are preferably the 3-position, 4-position or 5-position, and more preferably the 4-position.

In addition, examples of the first urea compound of the present invention include N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-ethyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-5-methyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-propyl-phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di- [3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[3-(p-xylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-xylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(1-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-propylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-isopropylbenzenesulfonyloxy)phenyl]urea, N, N'-di-[3-(p-t-butylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m,p-dimethoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-ethoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-propoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-butoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-kumi N,N'-di-[3-(p-cumylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-phenylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-phenylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-chlorobenzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, and the like, but are not limited thereto.

The second urea compound used in the present invention is represented by the following formula (Chemical Formula 3).

In the general formula (Chemical formula 3), R ² and R ⁴ to R ⁸ are defined as above. In the general formula (Chemical formula 3), R ⁴ to R ⁸ are preferably a hydrogen atom, an alkyl group, or an alkoxy group. In particular, R ⁴ , R ⁵ , R ⁷ , and R ⁸ are preferably a hydrogen atom, and R ⁶ is preferably a hydrogen atom or an alkyl group. R ⁶ is particularly preferably an alkyl group.
The alkyl group (including those included in an alkylcarbonyloxy group, an alkylcarbonylamino group, an alkylsulfonylamino group, a monoalkylamino group, and a dialkylamino group) and the aryl group (including those included in an aryloxy group, an arylcarbonyloxy group, an arylcarbonylamino group, an arylsulfonylamino group, and an arylamino group) are defined in the same manner as the alkyl group and the aryl group in the above general formula (Chemical Formula 2).
The alkoxy group is, for example, a linear, branched or alicyclic alkoxy group, and preferably has 1 to 12 carbon atoms.
The position of the —O—(CONH) _m —SO ₂ -substituted phenyl group in the benzene ring of general formula (Chemical formula 3) is preferably the 3rd, 4th or 5th position (the same applies to the following general formulae (Chemical formula 7) and (Chemical formula 8)).
In the general formula (Chemical Formula 3), m represents an integer of 0 to 2, preferably 0 to 1.

As the second urea compound of the present invention , a urea compound represented by the following general formula (Formula 8) is preferable .

一般式（化４）中、Ｒ^２、Ｒ^４～Ｒ^８は上記と同様に定義される。 The third urea compound used in the present invention is represented by the following formula (Chemical Formula 4).

In the general formula (Chemical Formula 4), R ² , R ⁴ to R ⁸ are defined as above.

The third urea compound is preferably N-[2-(3-phenylureido)phenyl]benzenesulfonamide, which is represented by the following formula and is available, for example, from Nippon Soda Co., Ltd. under the trade name NKK1304.

The content of the urea compound in the heat-sensitive recording layer of the present invention (solid content, total amount when a plurality of urea compounds are contained) is 1.0 to 70.0% by weight, preferably 5.0 to 65.0% by weight, more preferably 10.0 to 60.0 parts by weight.
The content of the first urea compound in the heat-sensitive recording layer of the present invention is 1.0 to 50.0% by weight, preferably 5.0 to 40.0% by weight, and the content of the second urea compound is 5.0 to 50.0% by weight, preferably 5.0 to 40.0% by weight.

The thermal recording layer of the present invention may use a color developer other than the first and second urea compounds. Examples of such color developers include inorganic acidic substances such as activated clay, attapulgite, colloidal silica, and aluminum silicate, 4,4'-isopropylidenediphenol, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 4,4'-dihydroxydiphenyl sulfide, hydroquinone monobenzyl ether, 4-hydroxybenzoate benzyl, 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, and 4-hydroxy-4'-isopropoxydiphenyl sulfone. 4-hydroxy-4'-n-propoxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 4-hydroxyphenyl-4'-benzyloxyphenyl sulfone, 3,4-dihydroxyphenyl-4'-methylphenyl sulfone, 1-[4-(4-hydroxyphenylsulfonyl)phenoxy]-4-[4-(4-isopropoxyphenylsulfonyl)phenoxy]butane, the phenol condensation composition described in JP-A-2003-154760, the aminobenzenesulfonamide derivative described in JP-A-8-59603, bis(4-hydroxyphenylthioethoxy)methane, 1,5-di(4-hydroxyphenyl) 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 1,4-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, 1,3-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, di(4-hydroxy-3-methylphenyl)sulfide, 2,2'-thiobis(3-tert-octylphenol), 2,2'-thiobis(4-tert-octylphenol), compounds described in WO02/081229 or JP2002-301873A, and N,N'-di-m-chlorophenyl Examples of the color developers include thiourea compounds such as nylthiourea, p-chlorobenzoic acid, stearyl gallate, bis[zinc 4-(n-octyloxycarbonylamino)salicylate] dihydrate, 4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid, and 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, and salts of these aromatic carboxylic acids with polyvalent metal salts such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin, and nickel, as well as antipyrine complexes of zinc thiocyanate and complex zinc salts of terephthalaldehyde acid and other aromatic carboxylic acids. These color developers can be used alone or in combination of two or more. 1-[4-(4-hydroxyphenylsulfonyl)phenoxy]-4-[4-(4-isopropoxyphenylsulfonyl)phenoxy]butane is available, for example, under the trade name JKY-214 manufactured by API Corporation, and the phenol condensation composition described in JP 2003-154760 A is available, for example, under the trade name JKY-224 manufactured by API Corporation. Compounds described in WO02/081229 and the like are available under the trade names NKK-395 and D-100 manufactured by Nippon Soda Co., Ltd. In addition, metal chelate-type color-developing components such as higher fatty acid metal double salts and polyvalent hydroxy aromatic compounds described in JP 10-258577 A can also be contained.

When the thermosensitive recording layer of the present invention contains a color developer other than the first and second urea compounds, the total content (solid content) of the first and second urea compounds used relative to the total color developers (including the first and second urea compounds) contained in the thermosensitive recording layer is preferably 50% by weight or more, more preferably 80% by weight or more, and even more preferably 90% by weight or more.

The leuco dye used in the present invention can be any dye known in the field of conventional pressure-sensitive or heat-sensitive recording paper, and is not particularly limited. However, triphenylmethane compounds, fluoran compounds, fluorene compounds, divinyl compounds, etc. are preferred. Specific examples of representative colorless or light-colored dyes (dye precursors) are shown below. These dye precursors may be used alone or in combination of two or more.

<Triphenylmethane leuco dyes>
3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (also known as crystal violet lactone), 3,3-bis(p-dimethylaminophenyl)phthalide (also known as malachite green lactone)

<Fluoran-based leuco dyes>
3-diethylamino-6-methylfluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-6-methyl-7-(o-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-(o-fluoroanilino)fluoran, 3-diethylamino-6-methyl-7-(m-methylanilino)fluoran, 3-diethylamino-6-methyl-7 -n-octylanilinofluoran, 3-diethylamino-6-methyl-7-n-octylaminofluoran, 3-diethylamino-6-methyl-7-benzylaminofluoran, 3-diethylamino-6-methyl-7-dibenzylaminofluoran, 3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-diethylamino-6-chloro-7-p-methylanilinofluoran, 3-diethylamino-6-ethoxyethyl-7-anilinofluoran, 3-diethylamino-7-methylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-diethylamino-7-(p-chloroanilino)fluoran, 3-diethylamino-7-(o-fluoroanilino)fluoran, 3-diethylamino-benzo[a]fluoran, 3-diethylamino-benzo[c]fluoran, 3-dibutylamino-6-methyl-fluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-(o,p-dimethylanilino)fluoran, 3-dibutylamino-6-methyl-7-(o-chloroanilino)fluoran, 3-dibutylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-dibutylamino-6-methyl-7-(o-fluoroanilino)fluoran fluoran, 3-dibutylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran, 3-dibutylamino-6-methyl-7-chlorofluoran, 3-dibutylamino-6-ethoxyethyl-7-anilinofluoran, 3-dibutylamino-6-chloro-7-anilinofluoran, 3-dibutylamino-6-methyl-7-p-methylanilinofluoran, 3-dibutylamino-7-(o-chloroanilino)fluoran, 3-dibutylamino-7-(o-fluoroanilino)fluoran, 3-di-n-pentylamino-6-methyl-7-anilinofluoran, 3-di-n-pentylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-di-n-pentylamino- 7-(m-trifluoromethylanilino)fluoran, 3-di-n-pentylamino-6-chloro-7-anilinofluoran, 3-di-n-pentylamino-7-(p-chloroanilino)fluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-xylamino)-6-methyl-7-(p-chloroanilino)fluoran, 3-(N-ethyl 1-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-chloro-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran, 3-cyclohexylamino-6-chlorofluoran, 2-(4-oxahexyl)-3-dimethylamino-6-methyl-7-anilinofluoran, 2-(4-oxahexyl)- 3-diethylamino-6-methyl-7-anilinofluoran, 2-(4-oxahexyl)-3-dipropylamino-6-methyl-7-anilinofluoran, 2-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-methoxy-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-chloro-3-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 2-chloro-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-nitro-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 2-amino-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 2-diethylamino No-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 2-phenyl-6-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 2-benzyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 2-hydroxy-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 3-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-dibutylaminophenyl)aminoanilinofluoran, 2,4-dimethyl-6-[(4-dimethylamino)anilino]-fluoran

<Fluorene-based leuco dyes>
3,6,6'-tris(dimethylamino)spiro[fluorene-9,3'-phthalide], 3,6,6'-tris(diethylamino)spiro[fluorene-9,3'-phthalide]

<Divinyl leuco dye>
3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrabromophthalide, 3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrachlorophthalide, 3,3-bis-[1,1-bis(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrabromophthalide, 3,3-bis-[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide

＜Other＞
3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-octyl-2-methylindol-3-yl)-4-azaphthalide, 3-(4-cyclohexylethylamino-2-methoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide, 3,6-bis(diethylamino)fluoran-γ-(3′-nitro)anilinolactam, 3,6-bis(diethylamino)fluoran-γ-(3′-nitro)anilinolactam, Fluoran-γ-(4'-nitro)anilinolactam, 1,1-bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-dinitrileethane, 1,1-bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2-β-naphthoylethane, 1,1-bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-diacetylethane, bis-[2,2,2',2'-tetrakis-(p-dimethylaminophenyl)-ethenyl]-methylmalonic acid dimethyl ester

The sensitizer used in the present invention may be a conventionally known sensitizer. Examples of such sensitizers include fatty acid amides such as stearic acid amide and palmitic acid amide, ethylene bisamide, montanic acid wax, polyethylene wax, 1,2-bis-(3-methylphenoxy)ethane, p-benzylbiphenyl, β-benzyloxynaphthalene, 4-biphenyl-p-tolyl ether, m-terphenyl, 1,2-diphenoxyethane, dibenzyl oxalate, di(p-chlorobenzyl) oxalate, di(p-methylbenzyl) oxalate, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, di-p-tolyl carbonate, phenyl-α- Examples of the sensitizers include naphthyl carbonate, 1,4-diethoxynaphthalene, 1-hydroxy-2-naphthoic acid phenyl ester, o-xylene-bis-(phenyl ether), 4-(m-methylphenoxymethyl)biphenyl, 4,4'-ethylenedioxy-bis-benzoic acid dibenzyl ester, dibenzoyloxymethane, 1,2-di(3-methylphenoxy)ethylene, bis[2-(4-methoxy-phenoxy)ethyl]ether, methyl p-nitrobenzoate, phenyl p-toluenesulfonate, o-toluenesulfonamide, and p-toluenesulfonamide. These sensitizers may be used alone or in combination of two or more.

Pigments that can be used in the present invention include kaolin, calcined kaolin, calcium carbonate, aluminum oxide, titanium oxide, magnesium carbonate, aluminum silicate, magnesium silicate, calcium silicate, aluminum hydroxide, silica, etc., and can be used in combination depending on the required quality.

Examples of binders used in the present invention include fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, acetoacetylated polyvinyl alcohol, carboxy-modified polyvinyl alcohol, amide-modified polyvinyl alcohol, sulfonic acid-modified polyvinyl alcohol, butyral-modified polyvinyl alcohol, olefin-modified polyvinyl alcohol, nitrile-modified polyvinyl alcohol, pyrrolidone-modified polyvinyl alcohol, silicone-modified polyvinyl alcohol, other modified polyvinyl alcohols, hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, styrene-maleic anhydride copolymer, styrene-butadiene copolymer, as well as cellulose derivatives such as ethyl cellulose and acetyl cellulose, casein, gum arabic, oxidized starch, etherified starch, dialdehyde starch, esterified starch, polyvinyl chloride, polyvinyl acetate, polyacrylamide, polyacrylic acid esters, polyvinyl butyral, polystyrene and copolymers thereof, polyamide resins, silicone resins, petroleum resins, terpene resins, ketone resins, and coumarone resins. These polymeric substances can be used by dissolving them in solvents such as water, alcohol, ketones, esters, and hydrocarbons, or by dispersing them in water or other media in an emulsified or paste-like form, and can also be used in combination depending on the required quality.

The lubricants used in the present invention include fatty acid metal salts such as zinc stearate and calcium stearate, waxes, silicone resins, and the like.

In the present invention, 4,4'-butylidene (6-t-butyl-3-methylphenol), 2,2'-di-t-butyl-5,5'-dimethyl-4,4'-sulfonyldiphenol, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, etc. can be added as stabilizers to improve the oil resistance of the image area within a range that does not impair the desired effect for the above problem. In addition, benzophenone-based or triazole-based ultraviolet absorbers, dispersants, defoamers, antioxidants, fluorescent dyes, etc. can be used.

The types and amounts of the leuco dye, developer, sensitizer, and other various components used in the thermal recording layer of the present invention are determined according to the required performance and recording suitability, and are not particularly limited, but typically, 0.5 to 10 parts by weight of developer, 0.1 to 10 parts by weight of sensitizer, 0.5 to 20 parts by weight of pigment, 0.01 to 10 parts by weight of stabilizer, and 0.01 to 10 parts by weight of other components are used per 1 part by weight of leuco dye. The binder is preferably about 5 to 25% by weight of the solid content of the thermal recording layer.

In the present invention, the leuco dye, developer, and materials added as necessary are pulverized to particle sizes of several microns or less using a grinding machine such as a ball mill, attritor, or sand grinder, or an appropriate emulsifying device, and a binder and various additives depending on the purpose are added to make a coating liquid. The solvent used for this coating liquid can be water or alcohol, and the solid content is about 20 to 40% by weight.

In the thermosensitive recording medium of the present invention, an undercoat layer is provided between the support and the thermosensitive recording layer.
The undercoat layer consists primarily of a binder and a pigment.
As the binder for use in the undercoat layer, the binders usable in the above-mentioned heat-sensitive recording layer can be appropriately used. These binders may be used alone or in combination of two or more kinds.

The pigment used in the undercoat layer contains plastic hollow particles. The plastic hollow particles used in the present invention are fine hollow particles that have a thermoplastic resin shell and contain air or other gases inside, and are already in a foamed state. Examples of thermoplastic resins include polystyrene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyacrylic acid esters, polyacrylonitrile, polybutadiene, or copolymers thereof. In particular, styrene-based resins such as polystyrene, acrylic resins such as polyacrylic acid esters and polyacrylonitrile, copolymers thereof, or copolymer resins mainly composed of polyvinylidene chloride and polyacrylonitrile are preferred. Such organic hollow particles are available as SX8782 manufactured by JSR Corporation, MH5055 and MH8108A manufactured by Nippon Zeon Co., Ltd., Ropeake HP-91 manufactured by Rohm & Haas Japan Co., Ltd., Microsphere manufactured by Matsumoto Yushi Co., Ltd., and the like.
The volumetric hollow ratio of the plastic hollow particles used in the present invention is preferably about 40 to 95%. By making the volumetric hollow ratio 40% or more, it is possible to improve the heat insulating properties and further increase the color development performance. On the other hand, by making it 95% or less, it is possible to increase the strength of the shell of the hollow particles, effectively maintain the hollow state, and easily obtain an undercoat layer with good surface strength. Here, the volumetric hollow ratio is a value calculated by (d3/D3) x 100. In this formula, d indicates the inner diameter of the organic hollow particles, and D indicates the outer diameter of the organic hollow particles.

The undercoat layer may contain a pigment other than the hollow plastic particles, and examples of the pigments that can be used include inorganic pigments such as calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, kaolin, calcined kaolin, clay, and talc, and organic pigments such as hollow plastic particles. These pigments may be used alone or in combination of two or more. As the pigment other than the hollow plastic particles, it is preferable to use calcined kaolin.
The content of the pigment in the undercoat layer is usually 50 to 95% by weight, preferably 70 to 90% by weight, based on the solid content of the undercoat layer. The content of the hollow plastic particles in the undercoat layer is 50% by weight or more, preferably 70 to 100% by weight, more preferably 80 to 100% by weight, based on the solid content of the pigment in the undercoat layer.
The coating solution for the undercoat layer may contain various auxiliary agents, such as dispersants, plasticizers, pH adjusters, defoamers, water retention agents, preservatives, coloring dyes, and ultraviolet protection agents, as required.

The thermosensitive recording medium of the present invention may further have a protective layer on the thermosensitive recording layer.
This protective layer mainly comprises a binder and a pigment, to which a crosslinking agent may further be added.
As the binder, any of the binders usable in the above-mentioned heat-sensitive recording layer can be appropriately used, but preferred are carboxy-modified polyvinyl alcohol and non-core-shell type acrylic resins having a glass transition point (Tg) higher than 50° C. These binders may be used alone or in combination of two or more.
Examples of the crosslinking agent include epichlorohydrin resins such as polyamine epichlorohydrin resins and polyamide epichlorohydrin resins, polyamide urea resins, polyalkylene polyamine resins, polyalkylene polyamide resins, polyamine polyurea resins, modified polyamine resins, modified polyamide resins, polyalkylene polyamine urea formalin resins, and polyalkylene polyamine polyamide polyurea resins, and other polyamine/polyamide resins, glyoxal, methylol melamine, melamine formaldehyde resins, melamine urea resins, potassium persulfate, ammonium persulfate, sodium persulfate, ferric chloride, magnesium chloride, borax, boric acid, alum, and ammonium chloride. It is preferable to incorporate an epichlorohydrin resin and a polyamine/polyamide resin as crosslinking agents in the protective layer, since this provides particularly good water resistance.
The amount of the binder or the total amount of the binder and pigment in the protective layer is usually 80.0 to 100.0% by weight, preferably 90.0 to 100.0% by weight, in terms of solid content. The amount of the binder is preferably about 30.0 to 300.0 parts by weight per 100 parts by weight of the pigment.
The coating solution for the protective layer may contain various auxiliary agents such as lubricants, stabilizers, ultraviolet absorbers, dispersants, antifoaming agents, antioxidants, fluorescent dyes, etc., which can be used in the above-mentioned heat-sensitive recording layer, as required.

In the present invention, the means for coating the thermosensitive recording layer and the coating layers other than the thermosensitive recording layer, i.e., the protective layer, undercoat layer, etc., is not particularly limited, and can be applied according to well-known conventional techniques. For example, an off-machine coater or an on-machine coater equipped with various coaters such as an air knife coater, a rod blade coater, a bent blade coater, a bevel blade coater, a roll coater, or a curtain coater can be appropriately selected and used.
The coating weights of the thermosensitive recording layer and the coating layers other than the thermosensitive recording layer are determined according to the required performance and recording suitability, and are not particularly limited. However, the typical coating weight of the thermosensitive recording layer is about 2 to 12 g/ ^m2 in solid content, and the coating weight of the protective layer is preferably 0.5 to 5.0 g/ ^m2 in solid content.
Furthermore, various known techniques in the field of thermal recording media can be added as necessary, such as smoothing treatment such as supercalendering after coating of each coating layer.

The following examples illustrate the present invention but are not intended to limit it. In each example and comparative example, "parts" means "parts by weight" and "%" means "% by weight" unless otherwise specified.

For the production of a thermal recording medium, each dispersion and coating liquid was prepared as follows.
[Preparation of each coating liquid]
The following compositions were mixed and dispersed to prepare coating solutions 1 to 4 for the undercoat layer.
<Coating solution 1 for undercoat layer>
Calcined kaolin (manufactured by BASF, product name: Ansilex 90) 40.0 parts Plastic hollow particles (manufactured by Nippon Zeon Co., Ltd., product name: Nipol
MH8108A, hollow ratio 50%, solid content 27%) 222.2 parts styrene-butadiene copolymer latex (manufactured by Nippon Zeon Co., Ltd.,
Product name: ST5526, solid content 48%) 10.0 parts <Undercoat layer coating liquid 2>
Calcined kaolin (Ansilex 90) 30.0 parts Plastic hollow particles (Nipol MH8108A) 259.3 parts Styrene-butadiene copolymer latex (ST5526) 10.0 parts <Coating liquid 3 for undercoat layer>
Calcined kaolin (Ansilex 90) 20.0 parts Plastic hollow particles (Nipol MH8108A) 296.3 parts Styrene-butadiene copolymer latex (ST5526) 10.0 parts <Coating liquid 4 for undercoat layer>
Calcined kaolin (Ansilex 90) 10.0 parts Plastic hollow particles (Nipol MH8108A) 333.3 parts Styrene-butadiene copolymer latex (ST5526) 10.0 parts

<Coating solution 5 for undercoat layer>
Plastic hollow particles (Nipol MH8108A) 370.0 parts Styrene-butadiene copolymer latex (ST5526) 10.0 parts <Coating liquid 6 for undercoat layer>
Calcined kaolin (Ansilex 90) 60.0 parts Plastic hollow particles (Nipol MH8108A) 148.1 parts Styrene-butadiene copolymer latex (ST5526) 10.0 parts <Coating liquid 7 for undercoat layer>
Calcined kaolin (Ansilex 90) 50.0 parts Plastic hollow particles (Nipol MH8108A) 185.1 parts Styrene-butadiene copolymer latex (ST5526) 10.0 parts Water 50.0 parts <Coating solution 8 for undercoat layer>
Calcined kaolin (Ansilex 90) 100.0 parts Styrene-butadiene copolymer latex (ST5526) 10.0 parts Water 50.0 parts

The developer dispersions (Liquids A1 to A5), leuco dye dispersion (Liquid B), and sensitizer dispersion (Liquid C) with the following formulations were each wet-ground separately in a sand grinder until the average particle size was 0.5 μm.

Developer dispersion liquid (A1 liquid)
N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea (hereinafter referred to as "urea compound 1") 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (manufactured by Kuraray Co., Ltd., trade name:
PVA117, solid content 10%) 5.0 parts Water 1.5 parts

完全ケン化型ポリビニルアルコール水溶液（ＰＶＡ１１７） ５．０部
水 １．５部
顕色剤分散液（Ａ３液）
Ｎ－［２－（３－フェニルウレイド）フェニル］ベンゼンスルホンアミド
（以下、「ウレア化合物３」という。） ６．０部
完全ケン化型ポリビニルアルコール水溶液（ＰＶＡ１１７） ５．０部
水 １．５部 Developer dispersion liquid (liquid A2)
A urea compound represented by the following chemical formula (9):
(hereinafter referred to as "urea compound 2") 6.0 parts

Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts
Developer dispersion liquid (A3 liquid)
N-[2-(3-phenylureido)phenyl]benzenesulfonamide (hereinafter referred to as "urea compound 3") 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts

完全ケン化型ポリビニルアルコール水溶液（ＰＶＡ１１７） ５．０部
水 １．５部 Developer dispersion liquid (A4 liquid)
Urea urethane compound represented by the chemical formula (13) (UU manufactured by Fine Ace Co., Ltd.) 6.0 parts

Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts

Developer dispersion liquid (A5 liquid)
4-Hydroxy-4'-isopropoxydiphenylsulfone (manufactured by Mitsubishi Chemical Corporation, trade name: NYDS) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts

Leuco dye dispersion (liquid B)
3-Dibutylamino-6-methyl-7-anilinofluoran (manufactured by Yamamoto Chemical Industries, Ltd., trade name: ODB-2) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts

Sensitizer dispersion (liquid C)
1,2-bis-(3-methylphenoxy)ethane (manufactured by Sankosha,
Product name: KS232) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts

Next, each dispersion was mixed in the following ratio to prepare a coating liquid for the heat-sensitive recording layer.
<Coating solution for thermal recording layer>
Developer dispersion (liquid A1) 36.0 parts Leuco dye dispersion (liquid B) 18.0 parts Sensitizer dispersion (liquid C) 5.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 25.0 parts

Next, the following components were mixed in the following proportions to prepare a coating solution for the protective layer.
<Protective Layer Coating Fluid>
Aluminum hydroxide dispersion (Martinsberg,
Product name: Martyfin OL, solid content 50%) 9.0 parts Carboxy-modified polyvinyl alcohol aqueous solution (manufactured by Kuraray Co., Ltd., product name:
KL318, polymerization degree: about 1800, saponification degree: 85 to 90 mol%,
Solid content: 10%) 30.0 parts Polyamide epichlorohydrin resin (manufactured by Seiko PMC Corporation, product name:
WS4030, solid content 25%) 4.0 parts Modified polyamine resin (manufactured by Taoka Chemical Co., Ltd., product name: Sumirez Resin SPI
-102A, solid content 45%) 2.2 parts zinc stearate (manufactured by Chukyo Yushi Co., Ltd., trade name: Hydrin Z-7-30,
Solid content 30%) 2.0 parts

[Example 1]
Coating solution 1 for undercoat layer was applied to one side of a support (fine paper with a basis weight of 47 g/ ^m2 ) by a bent blade method so that the coating amount was 10.0 g/ ^m2 in solid content, and then dried to obtain a coated paper for undercoat layer.
The coating liquid for the thermosensitive recording layer was applied onto the undercoat layer of this undercoat layer-coated paper by the rod blade method so that the coating amount was 6.0 g/ ^m2 in solid content, and then the coating was dried and treated with a super calendar to achieve a smoothness of 100 to 500 seconds to produce a thermosensitive recording medium.
[Example 2]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 3 was used instead of undercoat layer coating liquid 1, the amount of A1 liquid in the thermosensitive recording layer coating liquid was changed to 18 parts, and 18 parts of A4 liquid was added to the thermosensitive recording layer coating liquid.

[Example 3]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that Coating Liquid 3 for the undercoat layer was used instead of Coating Liquid 1 for the undercoat layer, and Liquid A2 was used instead of Liquid A1 in the Coating Liquid for the thermosensitive recording layer.
[Reference Example 4]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that Coating Liquid 3 for the undercoat layer was used instead of Coating Liquid 1 for the undercoat layer, and Liquid A3 was used instead of Liquid A1 in the coating liquid for the thermosensitive recording layer.
[Reference Example 5]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 4 was used instead of undercoat layer coating liquid 1, the amount of A1 liquid in the thermosensitive recording layer coating liquid was changed to 18 parts, and 18 parts of A2 liquid was added to the thermosensitive recording layer coating liquid.
[Reference Example 6]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that coating liquid 2 for the undercoat layer was used instead of coating liquid 1 for the undercoat layer, the amount of A1 liquid in the coating liquid for the thermosensitive recording layer was changed to 18 parts, and 18 parts of A3 liquid was added to the coating liquid for the thermosensitive recording layer.
[Reference Example 7]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 3 was used instead of undercoat layer coating liquid 1, the amount of A1 liquid in the thermosensitive recording layer coating liquid was changed to 18 parts, and 18 parts of A3 liquid was added to the thermosensitive recording layer coating liquid.

[Reference Example 8]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 5 was used instead of undercoat layer coating liquid 1, the amount of A1 liquid in the thermosensitive recording layer coating liquid was changed to 18 parts, and 18 parts of A3 liquid was added to the thermosensitive recording layer coating liquid.
[Reference Example 9]
A thermal recording medium was prepared in the same manner as in Reference Example 8, except that the protective layer coating liquid was applied by the rod blade method onto the thermal recording layer of the thermal recording layer-coated paper so that the coating amount was 2.0 g/ ^m2 in solid content, followed by drying and processing with a super calendar so that the smoothness was 100 to 500 seconds.
[Reference Example 10]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 3 was used instead of undercoat layer coating liquid 1, the amount of A1 liquid in the thermosensitive recording layer coating liquid was changed to 18 parts, and 9 parts of A3 liquid and 9 parts of A4 liquid were added to the thermosensitive recording layer coating liquid.
[Reference Example 11]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 3 was used instead of undercoat layer coating liquid 1, and A1 liquid in the thermosensitive recording layer coating liquid was not blended, and 18 parts of A2 liquid and 18 parts of A3 liquid were added.

[Comparative Example 1]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the undercoat layer coating liquid 1 was changed to the undercoat layer coating liquid 6.
[Comparative Example 2]
A thermosensitive recording medium was prepared in the same manner as in Example 3, except that no undercoat layer was provided.
[Comparative Example 3]
A thermosensitive recording medium was prepared in the same manner as in Example 3, except that the undercoat layer coating liquid 3 was changed to the undercoat layer coating liquid 8.
[Comparative Example 4]
A thermosensitive recording medium was prepared in the same manner as in Example 4, except that the undercoat layer coating liquid 3 was changed to the undercoat layer coating liquid 6.

[Comparative Example 5]
A thermosensitive recording medium was prepared in the same manner as in Example 5, except that the undercoat layer coating liquid 4 was changed to the undercoat layer coating liquid 6.
[Comparative Example 6]
A thermosensitive recording medium was prepared in the same manner as in Example 5, except that no undercoat layer was provided.
[Comparative Example 7]
A thermosensitive recording medium was prepared in the same manner as in Example 5, except that the undercoat layer coating liquid 4 was changed to the undercoat layer coating liquid 8.
[Comparative Example 8]
A thermosensitive recording medium was prepared in the same manner as in Example 6, except that no undercoat layer was provided.
[Comparative Example 9]
A thermosensitive recording medium was prepared in the same manner as in Example 6, except that the undercoat layer coating liquid 2 was changed to the undercoat layer coating liquid 8.
[Comparative Example 10]
A thermosensitive recording medium was prepared in the same manner as in Example 11, except that no undercoat layer was provided.

[Comparative Example 11]
A thermosensitive recording medium was prepared in the same manner as in Example 11, except that the undercoat layer coating liquid 3 was changed to the undercoat layer coating liquid 8.
[Comparative Example 12]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 1 was changed to undercoat layer coating liquid 7, and in the coating liquid for the thermosensitive recording layer, A1 liquid was not blended, and 36.0 parts of A5 liquid was blended.
[Comparative Example 13]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that undercoat layer coating liquid 1 was changed to undercoat layer coating liquid 3, and in the coating liquid for the thermosensitive recording layer, A1 liquid was not blended, and 36.0 parts of A5 liquid was blended.

The prepared thermosensitive recording medium was evaluated as follows.
<Color development performance (print density)>
A checkered pattern was printed on the prepared thermal recording medium at a printing speed of 50 mm/sec and an applied energy of 0.41 mJ/dot using a TH-PMD (thermal recording paper print tester, equipped with a thermal head manufactured by Kyocera Corporation) manufactured by Okura Electric Co., Ltd. The print density of the printed area was measured with a Macbeth densitometer (RD-914, using an amber filter) to evaluate the color development performance (print density).

<Suitable for high-speed printing>
On the prepared thermal recording medium, a barcode (CODE39) was printed vertically (the printer head movement direction and the barcode were perpendicular) and horizontally (the printer head movement direction and the barcode were parallel) using a Zebra 140XiIII label printer at a print level of +10 and a print speed of 30.4 cm/sec (12 in/sec).
Next, the printed barcode was read using a barcode verification machine (Honeywell, QCPC600, light source 640 nm) to evaluate the barcode readability. The evaluation results were recorded using the ANSI standard symbol grade.
Symbol grade: The barcode is divided into 10 parts vertically to the bars, and a reading test is conducted once for each part. The average value is expressed on a 5-level scale: A (excellent), B, C, D, F (poor).
<Heat resistance of blank areas>
The prepared thermal recording medium was treated under environmental conditions of 80° C. for 24 hours, and then left to stand under environmental conditions of 23° C.×50% RH for 3 hours.
The density of the non-printed areas (blank areas) was measured with a Macbeth densitometer (RD-914, using an amber filter), and the background color development value was calculated from the difference in the values before and after processing, and the heat resistance of the non-printed areas (blank areas) was evaluated according to the following criteria.
Background color value = (density of non-printed area after processing) - (density of non-printed area before processing)
Excellent: Background color value is less than 0.3. Passable: Background color value is 0.3 or more and less than 0.5. Unacceptable: Background color value is 0.5 or more.

<Plasticizer resistance>
A checkered pattern was printed on the prepared thermal recording medium using an Okura Electric TH-PMD (thermal recording paper print tester equipped with a Kyocera thermal head) at an applied energy of 0.41 mJ/dot and a printing speed of 50 mm/sec.
The printed thermosensitive recording medium was attached to a paper tube wrapped once with PVC wrap (Hi-wrap KMA, manufactured by Mitsui Chemicals), and then wrapped with PVC wrap three times. The medium was then left to stand at 40° C. for 24 hours.
The print density of the printed portion was measured with a Macbeth densitometer (RD-914, using an amber filter), and the residual rate was calculated from the values before and after the treatment to evaluate the plasticizer resistance.
Residual rate (%)=(print density of printed part after processing/print density of printed part before processing)×100
Excellent: Residual rate is 90% or more. Passable: Residual rate is 70% or more, but less than 90%. Poor: Residual rate is less than 70%. <Printing runnability (head residue resistance)>
A 10 cm long grid pattern was printed on the prepared thermal recording medium using a Sato label printer (Resply R-8), and the residue (head residue) adhering to the thermal head after printing was visually evaluated according to the following criteria.
Excellent: Almost no head residue was observed.
Pass: A small amount of head residue was observed, but the formed image was not blurred or missing, and was not of a level that would cause any practical problems.
Poor: A large amount of head residue was observed, and the formed image had some voids and faint spots.

The results are shown in the table below.

Claims (8)

A thermosensitive recording medium having an undercoat layer provided on a support and a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting color developer provided on the undercoat layer, wherein the thermosensitive recording layer contains a first urea compound represented by the following (1) or a second urea compound represented by the following (2) as the electron-accepting color developer, the undercoat layer contains a binder and a pigment, the pigment content being 50 to 95% by weight in terms of solid content, and the pigment contains hollow plastic particles, the pigment content being 50% by weight or more in terms of solid content,
the total content (solid content) of the first and second urea compounds in the thermal recording layer is 1.0 to 70.0% by weight;
A thermal recording medium , wherein the content (solid content) of the first urea compound in the thermal recording layer is 1.0 to 50.0% by weight, and the content (solid content) of the second urea compound is 5.0 to 50.0% by weight.
(1) The first urea compound is represented by the following general formula (Chemical Formula 5):

(In the formula, ^R2 represents a hydrogen atom or an alkyl group, ^R3 may be the same or different and is a group represented by the following formula (Chemical formula 11), and the position of ^R3 - _SO2 -O- in the benzene ring of general formula (Chemical formula 5) may be the same or different and is the 3rd, 4th or 5th position.)

(In the formula, R ⁴ to R ⁸ may be the same or different and each represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group.)
(2) The second urea compound is represented by the following general formula (Chemical Formula 3):

(In the formula, R ² , R ⁴ to R ⁸ are defined as above, and m represents an integer of 0 to 2), with the proviso that the second urea compound does not include a compound represented by the following general formula (Chemical Formula 7).

2. The thermal recording material according to claim 1, wherein the first urea compound is represented by the following general formula (Chemical Formula 6).

(In the formula, R ⁹ may be the same or different and represents an alkyl group or an alkoxy group, and o represents an integer of 0 to 3.) 3. The thermal recording material according to claim 2, wherein in the first urea compound, R ⁹ represents an alkyl group having 1 to 4 carbon atoms, o represents an integer of 0 to 1, and R ⁹ is located at the 4-position in the benzene ring. 4. The thermal recording medium according to claim 1 , wherein the second urea compound is represented by the following general formula (Chemical Formula 8) .

The thermosensitive recording medium according to any one of claims 1 to 4, wherein the thermosensitive recording layer contains a color developer other than the first urea compound and the second urea compound, and the total content (solid content) of the first urea compound and the second urea compound relative to the total color developer contained in the thermosensitive recording layer is 90% by weight or more . 6. The thermosensitive recording medium according to claim 1 , wherein the volumetric hollow ratio of the hollow plastic particles is 40 to 95%. The undercoat layer is
(i) The pigment contains 50 to 95% by weight of solid content, and the pigment contains 80 to 100% by weight of hollow plastic particles in terms of solid content, or
(ii) The heat-sensitive recording material according to claim 1 , wherein the pigment contains 70 to 90% by weight in terms of solid content, and the pigment contains 70 to 100% by weight in terms of solid content of hollow plastic particles. 8. The thermal recording material according to claim 1, wherein when the undercoat layer contains a pigment other than the plastic hollow particles, the pigment contains calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, kaolin, calcined kaolin, clay, or talc .